The Shrinking Glaciers of Kilimanjaro: Can Global Warming Be Blamed?

The Shining Mountain

What about Kilimanjaro? Tropical glacier-climate relations are different, but among them Kilimanjaro's glacial regime is unique. Its ice consists of an ice cap (up to 40 meters thick) sitting on the relatively flat summit plateau of its tallest volcanic peak, Kibo, about 5,700 to 5,800 meters above sea level and, below this, several slope glaciers. The slope glaciers extend down to about 5,200 meters (one, in a shady gully, extends to 4,800 meters). The ice cap is too thin to be deformed, and the plateau is too flat to allow for gliding. The summit's flanks are plenty steep—with angles averaging 35 degrees—but the slope glaciers move little compared with midlatitude, temperate glaciers. The slope glaciers gain and lose mass along their inclined surfaces. The plateau ice, by contrast, has two faces that each interact quite differently with the atmosphere and therefore with climate: near-horizontal surfaces and near-vertical cliffs, the latter forming the edges of the plateau ice.

What factors may explain the decline in Kilimanjaro's ice? Global warming is an obvious suspect, as it has been clearly implicated in glacial declines elsewhere, on the basis of both detailed mass-balance studies (for the few glaciers with such studies) and correlations between glacial length and air temperature (for many other glaciers). Rising air temperatures change the surface energy balance by enhancing sensible-heat transfer from atmosphere to ice, by increasing downward infrared radiation and finally by raising the ELA and hence expanding the area over which loss can occur. The first and only paper asserting that the glacier shrinkage on Kibo was associated with rising air temperatures was published in 2000 by Lonnie G. Thompson of Ohio State University and co-authors.

Another possible culprit is a decrease in accumulation combined with an increase in sublimation, both possibly driven by a change in the frequency and quantity of cloudiness and snowfall. This argument traces its roots to 19th-century European explorers, and has been substantially improved after field work by Kaser, Douglas K. Hardy of the Climate System Research Center at the University of Massachusetts, Amherst, Tharsis Hyera and Juliana Adosi of the Tanzania Meteorological Agency and others.

In 2001 Hardy had invited Kaser to join him and some television journalists in the filming of a documentary on the ice retreat on Kibo. For about a year and a half, Hardy's instruments had been deployed on the Kibo summit, measuring weather; Kaser had been studying tropical glaciers for almost a decade and a half. The team set up tents just below one of the most impressive ice cliffs that delineates the Northern Ice Field on its southern edge. During a full five days and nights on the plateau, we observed the ice and discussed the mechanisms that drive the changes, a discussion stimulated from time to time by penetrating questions from the two journalists. Kibo's volcanic ash provided a drawing board, and a ski pole served as the pencil as a picture of the regime of the glaciers on Kibo grew clearer. Thus was formed the basic hypothesis that still drives our research and that our subsequent field measurements of mass and energy balance have largely confirmed, one in which local air temperature and its changes would play only a minor role. Here is the evidence.